The present invention relates to apparatuses and methods for supplying power to LEDs.
FIG. 1 shows a typical schematic of a conventional power-factor-corrected (PFC) flyback converter 1 for driving light emitting diodes (“LEDs”). The PFC driver circuit 1 has advantages of a low number of components, a high power factor, a constant average output current to the LEDs, and a small transformer size. In the PFC driver circuit 1, an input stage includes an AC power supply 2 coupled to an EMI filter stage comprised of an inductor and two capacitors. The input AC signal is then full wave rectified by the diode bridge rectifier 6, which produces a full wave rectified signal for input to the transformer stage 8. The output of the transformer stage 8 is output to the LED(s) with a voltage level Vout.
In this type of converter, the input capacitance, CIN, is chosen to be small so that the input voltage is very close to a rectified sinusoidal wave. A PFC controller 10 is used in this circuit to adjust the input current so that the average input current follows from the input voltage. The PFC controller 10 regulates the output current to the LEDs. The power MOSFET 12 turns on and off to control application of the voltage in the transformer stage 8 to the LED(s).
The feedback control loop provided back to the PFC controller 10 by Current and voltage comparator 14 has a narrow bandwidth so that it is not affected by the double-utility-frequency ripple that appears at the LEDs. Table 1 shows the description of pin functions of a typical PFC controller, such as an L6562AT from STMicroelectronics. See http://www.st.com/stonline/products/literature/ds/15310.pdf, which is incorporated by reference herein. The internal structure of this known chip is shown in FIG. 2.
TABLE 1Pin descriptionNameDescriptionINVInverting input of the error amplifier. The information on theoutput voltage of the PFC pre-regulator is fed into this pinthrough a resistor divider.COMPOutput of the error amplifier. A compensation network is placedbetween this pin and INV to achieve stability of the voltagecontrol loop and ensure high power factor.MULTMain input to the multiplier. This pin is connected to therectified mains voltage via a resistor divider and providesthe sinusoidal reference to the current loop.CSInput to the pulse width modulation (PWM) comparator. Thecurrent flowing in the MOSFET is sensed through a resistor, theresulting voltage is applied to this pin and compared with aninternal sinusoidal-shaped reference, generated by themultiplier, to determine MOSFET turn-off.ZCDTransformer's demagnetization sensing input for transition-modeoperation. A negative-going edge triggers MOSFET turn-on.GDThe gate driver output for driving power MOSFET.VccSupply voltage of both the signal part of the IC and the gatedriver.
It is often desired to have a dimming function in supplying power for lighting apparatuses. One commonly used dimmer is the well-known triac dimmer. However, the conventional driving circuit shown in FIG. 1 has several problems that arise when used in conjunction with a triac dimmer to provide a dimming function when driving LEDs.
A triac dimmer reduces its load power by chopping the load voltage that drives the current to the load during each half-cycle. FIG. 3 shows the load voltage waveform of triac dimmers driving incandescent lamp loads as used in a circuit such as the one shown in FIG. 1. A triac dimmer works very well for resistive loads such as incandescent lamps, but it breaks down for other types of loads.
For example, if the load supplied by a triac dimmer are switch mode power supplies driving LEDs, the sharply increasing input voltage that occurs when the triac fires at each half cycle leads to the occurrence of current ringing that reverses several times during the half cycle and causes the triac to turn off. The triac will then be triggered to turn on again by the varying input voltage, leading to flicker in the LED. FIG. 4 is a waveform showing the current ringing that occurs in the current at the output of the triac dimmer when a triac dimmer is used to drive a PFC power supply such as the one shown in FIG. 1. The ringing occurs at all firing angles, and is worst at 90 degrees. In FIG. 4, the scale in the figure is 0.25 A/division vertically, and 250 μs/division horizontally.
In this configuration, when the dimmer's triac is not on, the PFC power supply does not draw any current and the input impedance of the load becomes very high. A high input impedance causes the internal RC timing circuit of the triac dimmer to work improperly, leading to a different firing angle for each AC line cycle. This problem also occurs at all firing angles.
FIG. 5 shows the rectified input voltage waveform of PFC power supply when it is being driven by a triac dimmer, that is, the voltage across Cin in FIG. 1. As shown in FIG. 5, the low voltage portion of the voltage waveform differs from one cycle to the next cycle. In FIG. 5, the scale is 50V per division in the vertical axis, and 5 ms per division in the horizontal axis.
As shown in FIG. 1, power is provided to the PFC controller 10 via terminal Vcc. If a triac dimmer were to be used with the driver circuit 1, when the triac dimmer is set to a dimmed position, this would result in a reduced average input voltage being applied to the PFC controller 10, causing a relatively slow startup of the PFC controller 10, or may even result in the PFC controller being unable to startup. FIG. 6 is a Vcc voltage waveform of a PFC power supply during startup that shows the instability of voltage supplied to the PFC controller during startup in the half cycle, when the triac dimmer is dimmed. In FIG. 6, the scale is 5V per division in the vertical axis, and 500 ms per division in the horizontal axis.
Since the efficiency of LEDs is higher than an incandescent lamp, at a given setting of a triac dimmer, the LEDs always appears brighter than an incandescent lamp.
Finally, a triac requires a minimum holding current, typically 30 to 50 mA, to stay on during the entire half cycle. If current falls below the holding current level, or if the current reverses, the triac will turn off. In order to maintain the holding current when the output current is dimmed down at low conduction angles, a minimum loading is required at the output to avoid chaotic operation of the triac dimmer. FIG. 7 shows the rectified input voltage is not stable and causes the LEDs to flash at a very dim setting of the triac dimmer. In FIG. 7, the scale is 50V per division in the vertical axis, and 5 ms per division in the horizontal axis.
Thus, there is a need for a PFC power supply circuit that is designed to function correctly with a triac dimmer to prevent ringing, prevent inconsistent timing, have a fast startup, provide the same dimming profile as an incandescent lamp, and prevent flashing at dim settings.